1. Field of the Invention
The present invention relates to an image transmission apparatus and an image transmission system, and, more particularly, to an image transmission apparatus and system which are capable of reducing the amount of image data to be transmitted during periods of congestion on the network connecting the data source and display terminal.
2. Description of the Related Art
In recent years, applications in which images are sent to display terminals have continued to increase in the art. In such an application, an image generated by a personal computer, for example, is sent via a wired or wireless network to a remote image display apparatus or terminal such as a projector or the like to be displayed thereby. Data generated by the personal computer are sent in the data format of a presentation software program to the remote image display terminal, which converts the data into image data and displays an image based on the image data.
Although the image may also be displayed on a display terminal directly associated with the personal computer generating the display data, the present invention specifically addresses the problem associated with the display on the remote display apparatus that is connected via the network. Moreover, there may not even be a local display terminal in an application configuration such as when a network server serves as the source of image data to be transmitted to the remote image display terminal.
According to such a process, the remote image display terminal needs to have a component such as a CD-ROM drive or the like for updating the presentation software program when an upgraded version is released. However, the additional component makes it difficult to meet requirements for smaller and less expensive image display terminals.
One solution is that the personal computer (which typically incorporates a CD-ROM drive) converts the data into image data and sends the image data to the image display terminal. However, it takes several seconds to transmit the image data through an ordinary network to the remote image display terminal, because the amount of image data is generally quite large.
There has been disclosed a process for shortening the transmission time of such image data by dividing the image data into blocks of bit map data. If the number of colors of the bit map data is small, then the number of indices is reduced, the bit map data are converted into indices with fewer bits, and the index data are compressed and sent. See Japanese Patent Laid-open Publication No. 74173/98 for details.
According to another disclosed system, image data are divided into blocks. Then portions of an image that have changed are detected in order to be displayed, and only image data which correspond to the blocks in which the change was detected are sent to update the displayed image, thereby reducing the amount of transmitted data. See Japanese Patent Laid-open Publication No. 50694/2003 for details.
However, with the systems disclosed above, it was difficult to control the amount of transmitted image data such that transmission delay was reduced between an image transmission apparatus and an image reception apparatus, depending on the transmission capability of a network. To control the amount of image data to be transmitted to reduce transmission delay, it was necessary to compress the image data at a high compression ratio in order to reduce the amount of image data to be transmitted, which lowers the quality of the displayed image.
However, if the image data were to be continuously compressed at a compression ratio to reduce transmission delay, the network bandwidth was not effectively used, and the quality of displayed images was degraded.
When an image to be displayed was frequently changed, in part or entirely, the conventional image transmission apparatus had to send image data for an entire image each time the image was changed, and hence needed to send a greatly increased amount of image data. Even if the amount of image data to be transmitted was reduced by a general image compression process, when image data were sent in an amount exceeding the transmission capability of the network, the transmission delay of received image data increased.
Under such a network, if the amount of image data to be transmitted was reduced by a high compression process to match the network bandwidth in order to improve the response time for displaying received image data, then the quality of displayed images was greatly degraded.